The present invention relates to a method for cleaning the surface of a semiconductor substrate in one of the steps of manufacturing a semiconductor device.
In general, the surface of a silicon substrate is cleaned before element layers are formed on the substrate Known as a method for cleaning substrates is the method RCA proposed in 1970. This method consists of SC-1 cleaning for removing particles and the like and SC-2 cleaning for removing metal impurities and the like.
The SC-1 cleaning is performed by using a cleaning liquid heated to 70 to 80xc2x0 C. and composed of ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) and pure water (H2O) mixed in the ratio of 1:1:5 vol. The SC-2 cleaning is effected by using a cleaning liquid composed of hydrochloric acid (HCl), hydrogen peroxide and pure water mixed in the ratio of 1:1:1:5 vol.
The SC-1 cleaning is performed mainly for removing organic substance or dust from the surface of a silicon substrate. The above-mentioned cleaning liquid is heated to 70 to 80xc2x0 C. The substrate is soaked in the liquid for about 5 to 10 minutes. Thereafter, the substrate is rinsed and dried.
The cleaning liquid etches the surface of the substrate since it contains ammonium hydroxide which is alkali. Thus, in the SC-1 cleaning, particles and the like are removed from the surface of the substrate due to lift-off effect.
It is reported that as shown in FIG. 6, the higher the ammonium hydroxide concentration in the cleaning liquid and the higher the etching rate of the surface of the substrate, the larger the amount of particles removed in the S-C 1 cleaning (M. Meuris, Microcontamination [1992]).
However, it is also reported that as shown in FIG. 7, the higher the ammonium hydroxide concentration, the greater the roughness (micro-roughness) of the surface of the substrate due to anisotropic etching, degrading the gate oxide film (T. Ohmi, ECS [1992]).
In the conventional cleaning method, hydrogen peroxide is decomposed, changing the mixing ratio of hydrogen peroxide to ammonium hydroxide. The rate of the hydrogen peroxide decomposition depends on the impurity concentration in the cleaning liquid, also. Thus, the decomposition accelerates if the cleaning liquid contains iron (Fe), copper (Cu) or the like. Therefore, it is difficult to control the mixing ratio. Furthermore, as is known in the art, iron or aluminum (Al), if any contained in the ammonium hydroxide or hydrogen peroxide, adheres to the surface of the substrate.
Therefore, the amount of particles removed in the SC-1 cleaning depends on the ammonium hydroxide concentration.
To summarize, the SC-1 cleaning has the following disadvantages:
First, the surface of the semiconductor substrate is greatly roughed (increase of micro-roughness), and the gate oxide film is degraded.
Second, the mixing ratio of hydrogen peroxide to ammonium hydroxide varies since the hydrogen peroxide is decomposed during cleaning. Furthermore, the cleaning liquid accelerates decomposition of the hydrogen peroxide if it contains iron, copper or the like. Therefore, it is difficult to control the mixing ratio of the cleaning liquid.
The object of the present invention is to provide a method for cleaning a semiconductor substrate, in which the degree to which the surface is roughed due to etching is reduced, metal is prevented from adhering to the surface, and the mixing ratio of a cleaning liquid can be easily controlled.
The method of the present invention comprises a step of applying ozone water to the surface of a semiconductor substrate set in a cleaning chamber, and a step of generating a cleaning liquid in which an alkali liquid and ozone water are mixed at a desired ratio, after a lapse of a predetermined time period, and applying the cleaning liquid to the surface of the semiconductor substrate. By virtue of those features, an oxide film is formed on the surface of the substrate due to the ozone water, and reduces the degree to which the surface of the substrate is roughed when the substrate is cleaned by the cleaning liquid. As a result, the surface of the substrate can be kept flatter than in the conventional cleaning method.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.